1. Field of the Invention
This invention relates to a catalyst, to its production and to its use for the polymerization of conjugated dienes, more particularly butadiene, in the gas phase.
2. Description of the Related Art
Polybutadiene with a high percentage of cis-1,4-units has been produced on an industrial scale for some time and is used for the production of tires and other rubber products. The polymerization is carried out in the liquid phase using various catalyst systems. One particularly advantageous catalyst system for the production of polybutadiewne with a high percentage of cis-1,4-units is described in European patent 11 184. The catalyst system described in this document, which is used for the polymerization of butadiene in solution, consists of a carboxylate of the rare earths, an aluminium tri-alkyl and/or alkyl aluminium hydride and another Lewis acid.
The polymerization of conjugated dienes in solution has the disadvantage that, during the removal of the unreacted monomer and the solvent from the polymer formed, low molecular weight compounds can enter the environment through waste gases and wastewater and have to be disposed of accordingly.
It is also known that the polymerization of conjugated dienes can be carried out without the addition of solvents in the liquid monomers. Unfortunately, this process has the disadvantage that the polymerization reaction is accompanied by the release of a large amount of heat which is difficult to control and, accordingly, represents a certain potential danger. In addition, the separation of the polymers from the monomers again involves the risk of environmental pollution.
In recent years, the gas phase process has proved to be particularly advantageous for the production of polyethylene and polypropylene in particular and has been applied on an industrial scale. The environmental advantages of the gas phase process are attributable in particular to the fact that no solvents are used and emissions and wastewater pollution can be reduced.
Hitherto, there has been no known process for carrying out the polymerization of conjugated dienes, more particularly polybutadiene, directly from the gas phase. One reason for this may be that the Ziegler-Natta catalysts based on titanium, cobalt, nickel or neodymium used for the solution polymerization of conjugated dienes are not readily suitable for polymerization in the gas phase, particularly on account of the poor productivity, i.e. the small yield of polymer which can be obtained with a certain quantity of catalyst. Thus, the catalyst described in EP 11 184 is unsuitable for the polymerization of dienes, more particularly butadiene, in the gas phase to form polybutadiene with a high percentage of cis-1,4-units on account of the fact that its activity falls dramatically after only a short time in a gas phase polymerization (see comparison test).